Synchronous Presentation of Chronic Myelomonocytic Leukemia and Multiple Myeloma in a Treatment-Naïve Patient

Chronic myelomonocytic leukemia (CMML) is a clonal myelodysplastic/myeloproliferative neoplasm characterized by persistent peripheral blood monocytosis and dysplastic bone marrow morphology. Multiple myeloma (MM) is a plasma cell malignancy defined by clonal plasma cell proliferation and myeloma-defining events. The synchronous occurrence of CMML and MM is exceptionally rare, particularly in treatment-naïve and relatively young patients.

We present a 42-year-old-male presenting with progressive fatigue and leukocytosis. Laboratory evaluation revealed marked absolute monocytosis (16.4 × 10⁹/L), anemia, and neutropenia. Bone marrow examination demonstrated an expanded abnormal monocytic population, further confirmed by multiparametric flow cytometry. Conventional cytogenetic analysis revealed monosomy 7, supporting the presence of a clonal myeloid neoplasm consistent with CMML. Concurrently, bone marrow biopsy showed 10%-15% plasma cells. Serum protein electrophoresis identified an IgG kappa monoclonal paraprotein (3.5 g/dL), and imaging showed a lytic bone lesion and biopsy-provided extramedullary plasmacytoma, meeting the diagnostic criteria of symptomatic MM. The patient has no prior exposure to cytotoxic therapy. He received one cycle of azacitidine with subsequent worsening cytopenias, underscoring therapeutic complexity.

This case describes a rare concurrent presentation of CMML and MM in a young, treatment-naïve patient. The combination of persistent monocytosis, dysplastic marrow findings, and monosomy 7 supported the presence of clonal myeloid process coexisting with plasma cell myeloma. Recognizing concurrent hematologic malignancies is essential, as diagnostic overlap and treatment interactions present substantial management challenges. Additional molecular characterization of similar cases may elucidate potential pathogenetic connections and guide tailored therapeutic approaches.